The present invention relates to cationic electrodepositable compositions and, more particularly, to cationic electrodepositable compositions that are resistant to effects of biodegradation.
Electrodeposition as a coating application method involves deposition of a film-forming composition under the influence of an applied electrical potential. Advantages of electrodeposition over non-electrophoretic coating processes include increased paint utilization, outstanding corrosion protection and low environmental contamination. Since its introduction in 1972, cationic electrodeposition has steadily gained in popularity over anionic electrodeposition and today is by far the most prevalent method of electrodeposition. Throughout the world, more than 80 percent of all motor vehicles produced are given a primer coating by cationic electrodeposition.
In preparing the electrodepositable composition used in the cationic electrodeposition process, a resinous binder which contains basic groups, such as basic nitrogen groups, is neutralized with an acid. The resultant cationic resin is dispersed in water and combined with pigment and other additives normally used in the cationic electrodeposition process to form a paint. The neutralizing acids may include organic acids such as acetic acid and lactic acid as well as inorganic acids such as sulfamic.
The electrodeposition process involves immersing an electroconductive substrate into a bath of an aqueous electrocoating composition, the substrate serving as a charged electrode in an electrical circuit comprising the electrode and an oppositely charged counter-electrode. In the case of a cationic electrocoat composition, the workpiece serves as a cathode. Sufficient electrical current is applied between the electrodes to deposit a substantially continuous, adherent film of the electrocoating composition onto the surface of the electroconductive substrate. The electrocoated substrate is then conveyed to a rinsing operation where it is rinsed with an aqueous rinsing composition. Typical rinsing operations have multiple stages which can include closed loop spray and/or dip applications such as are described below. For example, in a spray rinse application the electrocoated substrate exits the electrocoating tank and is conveyed over a rinse tank while an aqueous rinsing composition is spray applied to the electrocoated surfaces of the substrate. Excess rinsing composition is permitted to drain from the substrate into the rinse tank below. The rinsing composition is then recirculated to the spraying apparatus for subsequent spray applications.
Recirculating the coating or rinsing compositions is both economically and environmentally desirable. However, the combination of organic nutrients, warmth, aeration and recirculation in an aqueous coating system creates an environment conducive to bacterial and fungal growth. These microorganisms, if left unchecked, can adversely affect the quality and appearance of the electrodeposited coating. Microorganisms present in the coating or rinsing compositions can cause pH shifts, particulate xe2x80x9cdirtxe2x80x9d deposition and biofouling, which detrimentally affect the appearance of the coating and reduce system performance. Organic acids, such as lactic and acetic acids, commonly used to neutralize the basic groups of the cationic electrodeposition composition, for such microorganisms are a major nutrient source.
Also, ethylene glycol ether alcohols can suppress microorganism growth in electrocoating compositions, but are undesirable ecologically.
A microbiocide composition containing a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one (commercially available as KAYTHON(copyright) LX from Rohm and Haas Co.) has been used commercially in electrodeposition coatings and rinse compositions as the sole microorganism control composition. Although effective for inhibiting and/or controlling the growth of microorganisms in such systems, this microbiocide is relatively expensive and can cause a rougher appearance than a coating composition without this microbiocide. Moreover, such microbiocide compositions can contain, as inert ingredients, metal salts such as magnesium nitrate and magnesium chloride. The presence of metal ions of these salts in electrodeposition systems is undesirable because the metals can cause coating defects due to gas generation at the cathode. Furthermore, such a microbiocide typically is not included as a component in the coating composition, but, rather, is added to the electrodeposition system in an assembly plant setting. Microbiocides can lose their effectiveness over time as they are depleted from the bath and constant replenishment is necessary. Moreover, some of the microbiocides discussed above can require special handling and disposal means.
U.S. Pat. No. 6,017,431 discloses sulfamic acid (an inorganic acid) as a neutralizing agent for cationic electrocoating compositions and for the adjustment of pH of the electrodeposition bath compositions containing these compositions. Such electrodeposition baths are more resistant to the adverse effects of microorganism growth when the amount of sulfamic acid in the electrodepositable composition is greater than 90 up to 100 equivalent weight percent. However, due to certain processing issues which can arise during the preparation of electrodeposition composition components containing sulfamic acid as the neutralizing agent, the inclusion of an organic acid in such electrodepositable compositions often is desirable. As mentioned above, however, organic acids, which are present to rectify these difficulties, can be consumed by bacteria. Moreover, in such cases, the indigestible sulfamic acid can be post-added to the electrodepositable composition to replace the organic acids consumed by bacteria.
In view of the foregoing, a need exists for an inherently biodegradation resistant electrodepositable coating composition that requires minimal maintenance. The elimination of the necessity to handle toxic microbiocides that often are used in electrodeposition baths neutralized with organic acids is also desirable. It was surprising to find that cationic electrodeposition compositions containing certain xcex1-alkoxycarboxylic acids exhibit improved resistance to biodegradation with use of little or no microbiocide.
In accordance with the present invention, a biodegradation resistant cationic electrodeposition composition is provided. The biodegradation resistant cationic electrodeposition composition comprises a resinous phase dispersed in an aqueous medium. The resinous phase comprises an electrodepositable resin having cationic onium salt groups which have been at least partially solubilized with an xcex1-alkoxycarboxylic acid of formula (I), 
where R is C1 to C6 alkyl or aryl and Rxe2x80x2 is H or C1 to C2 alkyl.
Also provided is a method of electrocoating a conductive substrate serving as a cathode in an electrical circuit comprising the cathode and an anode which are immersed in an aqueous electrodeposition bath comprising the biodegradation resistant cationic electrodepositable composition described above, and substrates coated by the method.
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term xe2x80x9cabout.xe2x80x9d Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of xe2x80x9c1 to 10xe2x80x9d is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.
As stated above, the present invention is directed to a biodegradation resistant cationic electrodepositable composition comprising a resinous phase dispersed in an aqueous medium. The resinous phase comprises an electrodepositable resin having cationic onium salt groups. The cationic onium salt groups are at least partially solubilized, or, where applicable, neutralized, with an xcex1-alkoxycarboxylic acid of the Structure (I) above.
As used herein, by xe2x80x9cbiodegradation resistant electrodepositable compositionxe2x80x9d is a composition which is resistant to the growth of microorganisms such as bacteria, algae, fungi and the like, which can cause system and coating deficiencies such as those discussed above.
The acids used in preparing the electrodepositable composition of the present invention are xcex1-alkoxycarboxylic acid of the general structure (I): 
where R is C1 to C25, typically C1 to C6, alkyl or aryl, and Rxe2x80x2 is H or C1 to C2 alkyl.
By xe2x80x9calkylxe2x80x9d is meant acyclic or cyclic, linear or branched alkyl groups having a carbon chain length of from C1 to C25. Optionally, the alkyl groups can contain heteroatoms, typically oxygen or nitrogen. Nonlimiting examples of such alkyl groups include methyl, ethyl, propyl, isopropyl, and butyl groups.
The xcex1-alkoxycarboxylic acid should be sufficiently water-soluble, so as to render the cationic resin composition dispersible in aqueous media. Nonlimiting examples xcex1-alkoxycarboxylic acids suitable for use in the present invention include methoxyacetic acid, ethoxyacetic acid, butoxyacetic acid, phenoxyacetic acid, 2-(2-methoxyethoxy)acetic acid and 2-[2-(methoxyethoxy)ethoxy]acetic acid.
In the biodegradation resistant cationic electrodepositable composition of the present invention, the xcex1-alkoxycarboxylic acid is used to at least partially solubilize (or, where applicable, neutralize) the cationic resin which comprises onium salt groups (e.g., ammonium, sulfonium and phosphonium groups). The degree of solubilization or neutralization depends upon the particular cationic resin included in the composition. In general, the degree of solubilization or neutralization is such that enough cationic groups are formed to provide a stable dispersion of the resin in aqueous media. By xe2x80x9cstable dispersionxe2x80x9d is meant one that does not settle or one that is easily redispersible if some settling occurs.
In general, the xcex1-alkoxycarboxylic acid is used in amounts such that the electrodepositable composition typically has a pH from 5.0 to 7.0, often from 5.5 to 6.5, and such that the resinous phase will migrate to and electrodeposit on the cathode (i.e., substrate) under the voltage imposed during the electrodeposition process. As previously discussed, the xcex1-alkoxycarboxylic acid can be used in the electrodepositable compositions of the present invention as the sole solubilizing agent for the cationic onium salt group-containing resin(s) or in conjunction with a conventional solubilizing acid.
Typically the xcex1-alkoxycarboxylic acid(s) is used in the electrodepositable compositions of the present invention in amount sufficient to solubilize 5 to 100 percent of the cationic onium salt groups contained in the cationic resin(s). If employed, conventional solubilizing agents, for example, acetic acid, lactic acid, and/or sulfamic acids, can be used in the electrodepositable compositions of the present invention in an amount sufficient to solubilize up to 95 percent of the cationic onium salt groups contained in the cationic resin(s). In any event, the percentage of cationic onium salt groups solubilized with the xcex1-alkoxycarboxylic acid(s) either solely or in combination with one or more conventional solubilizing agents typically is sufficient to ensure that the degree of solubilization/neutralization is at least 20 percent of the total theoretical solubilization/neutralization equivalent. As used herein and in the claims, by xe2x80x9csolubilizationxe2x80x9d and like terms is meant both total solubilization and total neutralization, as well as partial solublization and partial neutralization.
The method by which cationic onium salt groups are formed is dependent upon the type of onium salt group desired in the cationic resin. For example, the xcex1-alkoxycarboxylic acid can be used to at least partially solubilize a resin containing basic groups such as basic nitrogen groups. The basic nitrogen groups can be derived from the reaction of amines, such as primary or secondary amines, with epoxy groups.
Generally, the solubilization reaction can be conducted by adding the xcex1-alkoxycarboxylic acid to the resin with agitation, and subsequently dispersing the solubilized resin into an aqueous medium. Alternatively, a solution of the xcex1-alkoxycarboxylic acid in aqueous medium first can be prepared, and the resin can be added under agitation to the xcex1-alkoxycarboxylic solution to form the dispersion. The solubilization reaction can take place under relatively mild conditions, at temperatures ranging from 25xc2x0 C. to 70xc2x0 C.
If the cationic electrodepositable resin is only partially solubilized with one or more xcex1-alkoxycarboxylic acids, all or part of the remaining cationic onium salt groups can be solubilized with a conventional solubilizing acid, for example, lactic acid, sulfamic acid, formic acid and/or acetic acid. Because of their tendency to resist the effects of microorganism growth, inorganic acids, such as sulfamic acids, typically are employed for this purpose.
Also, for purposes of the present invention, it should be understood that any of the resinous components comprising the electrodepositable composition formulation which contain cationic salt groups can be at least partially solubilized with one or more xcex1-alkoxycarboxylic acid such as those described above. For example, as aforementioned, in addition to the main film-forming cationic resin, cationic electrodepositable compositions often include one or more cationic salt group-containing pigment grinding resins, the cationic salt groups of which can be at least partially solubilized with one or more a-alkoxycarboxylic acids.
Additionally, it is well known in the art to use conventional acids such as those described above, to maintain the electrodeposition bath pH. Such acids can be added directly to the bath. The xcex1-alkoxycarboxylic acids useful in the compositions of the present invention, such as those described above, can be directly added to the electrodeposition bath in addition to or in lieu of these conventional acids for this purpose.
The cationic electrodepositable resin, which usually contains active hydrogen groups, can be any suitable cationic resin known to those skilled in the art. The cationic electrodepositable resin, and, if included, a curing agent, typically constitute the main film-forming vehicle of the electrodepositable composition.
Examples of such cationic film-forming resins include amine salt group-containing resins such as the acid-solubilized reaction products of polyepoxides and primary or secondary amines such as those described in U.S. Pat. Nos. 3,663,389; 3,984,299; 3,947,338; and 3,947,339. Usually, these amine salt group-containing resins are used in combination with a blocked isocyanate curing agent. The isocyanate can be fully blocked as described in the aforementioned U.S. Pat. No. 3,984,299 or the isocyanate can be partially blocked and reacted with the resin backbone such as described in U.S. Pat. No. 3,947,338. Also, one-component compositions as described in U.S. Pat. No. 4,134,866 and DE-OS No. 2,707,405 can be used as the film-forming resin.
In one embodiment of the present invention, the cationic film-forming resins also can comprise cationic acrylic resins such as those described in U.S. Pat. Nos. 3,455,806 and 3,928,157. In an alternative embodiment of the present invention, the cationic electrodepositable resin comprises quaternary ammonium salt groups which are at least partially solubilized with an xcex1-alkoxy carboxylic acid of structure (I) above. Non-limiting examples of these resins are those which are formed from reacting an organic polyepoxide with a tertiary amine salt. Such resins are described in U.S. Pat. Nos. 3,962,165; 3,975,346; and 4,001,101. In another embodiment of the present invention, the cationic electrodepositable resin comprises ternary sulfonium salt groups which are at least partially solubilized with an xcex1-alkoxy carboxylic acid of the structure (I) above. In yet another embodiment of the present invention, the electrodepositable cationic resin comprises quaternary phosphonium salt-groups. Such resins are described in U.S. Pat. Nos. 3,793,278 and 3,984,922, respectively. The quaternary phosphonium salt groups are then at least partially solubilized with the xcex1-alkoxycarboxilyc acids described above. Also useful are the film-forming resins which cure via transesterification such as described in European Application No. 12463. Further, cationic compositions prepared from Mannich bases such as described in U.S. Pat. No. 4,134,932 can be used.
The cationic electrodepositable resin described above can be present in the electrodeposition bath of the invention in amounts ranging from 1 to 60 percent by weight, typically 5 to 25 based on total weight of the electrodeposition bath.
As mentioned above, the resinous phase of the electrodeposition bath of the present invention can further comprise a curing agent adapted to react with the active hydrogen groups of the cationic electrodepositable resin described immediately above. Blocked polyisocyanate curing agents typically are used. The polyisocyanates can be fully blocked as described in U.S. Pat. No. 3,984,299 column 1 lines 1 to 68, column 2 and column 3 lines 1 to 15, or partially blocked and reacted with the polymer backbone as described in U.S. Pat. No. 3,947,338 column 2 lines 65 to 68, column 3 and column 4 lines 1 to 30, which are incorporated by reference herein. By xe2x80x9cblockedxe2x80x9d is meant that the isocyanate groups have been reacted with a blocking agent so that the resultant blocked isocyanate group is stable to active hydrogens at ambient temperature but reactive with active hydrogens in the film forming polymer at elevated temperatures, usually between 90xc2x0 C. and 200xc2x0 C.
Suitable polyisocyanates include aromatic and aliphatic polyisocyanates, including cycloaliphatic polyisocyanates and representative examples include diphenylmethane-4,4xe2x80x2-diisocyanate (MDI), 2,4- or 2,6-toluene diisocyanate (TDI), including mixtures thereof, p-phenylene diisocyanate, tetramethylene and hexamethylene diisocyanates, dicyclohexylmethane-4,4xe2x80x2-diisocyanate, isophorone diisocyanate, mixtures of phenylmethane-4,4xe2x80x2-diisocyanate and polymethylene polyphenylisocyanate. Higher polyisocyanates such as triisocyanates can be used. An example would include triphenylmethane-4,4xe2x80x2,4xe2x80x3-triisocyanate. Isocyanate prepolymers with polyols such as neopentyl glycol and trimethylolpropane and with polymeric polyols such as polycaprolactone diols and triols (NCO/OH equivalent ratio greater than 1) can also be used.
The polyisocyanate curing agents are typically utilized in conjunction with the active hydrogen containing cationic electrodepositable resin in amounts ranging from 5 percent to 60 percent by weight, preferably from 20 percent to 50 percent by weight, the percentages based on the total weight of the resin solids of the electrodeposition bath.
The aqueous compositions of the present invention are in the form of an aqueous dispersion. The term xe2x80x9cdispersionxe2x80x9d is believed to be a two-phase transparent, translucent or opaque resinous system in which the resin is in the dispersed phase and the water is in the continuous phase. The average particle size of the resinous phase is generally less than 1.0 and usually less than 0.5 microns, preferably less than 0.15 micron.
The concentration of the resinous phase in the aqueous medium is at least 1 and usually from 2 to 60 percent by weight based on total weight of the aqueous dispersion. When the compositions of the present invention are in the form of resin concentrates, they generally have a resin solids content of 20 to 60 percent by weight based on weight of the aqueous dispersion.
Electrodeposition baths of the invention are typically supplied as two components: (1) a clear resin feed, which includes generally the active hydrogen-containing ionic electrodepositable resin, i.e., the main film-forming polymer, the curing agent, and any additional water-dispersible, non-pigmented components; and (2) a pigment paste, which generally includes one or more pigments, a water-dispersible grind resin which can be the same or different from the main-film forming polymer, and, optionally, additives such as wetting or dispersing aids and corrosion inhibitors. Electrodeposition bath components (1) and (2) are dispersed in an aqueous medium which comprises water and, usually, coalescing solvents.
The electrodeposition bath of the present invention has a resin solids content usually within the range of 5 to 25 percent by weight based on total weight of the electrodeposition bath.
As aforementioned, besides water, the aqueous medium may contain a coalescing solvent. Useful coalescing solvents include hydrocarbons, alcohols, esters, ethers and ketones. The preferred coalescing solvents include alcohols, polyols and ketones. Specific coalescing solvents include isopropanol, butanol, 2-ethylhexanol, isophorone, 2-methoxypentanone, ethylene and propylene glycol and the monoethyl, monobutyl and monohexyl ethers of ethylene glycol. The amount of coalescing solvent is generally between about 0.01 and 25 percent and when used, preferably from about 0.05 to about 5 percent by weight based on total weight of the aqueous medium.
As discussed above, a pigment composition and, if desired, various additives such as surfactants, wetting agents, corrosion inhibitors, such as the soluble salts of bismuth or lead, or catalyst can be included in the dispersion. The pigment composition may be of the conventional type comprising pigments, for example, iron oxides, strontium chromate, carbon black, coal dust, titanium dioxide, talc, barium sulfate, as well as color pigments such as cadmium yellow, cadmium red, chromium yellow and the like. The pigment content of the dispersion is usually expressed as a pigment-to-resin ratio. In the practice of the invention, when pigment is employed, the pigment-to-resin ratio is usually within the range of about 0.02 to 1:1. The other additives mentioned above are usually in the dispersion in amounts of about 0.01 to 3 percent by weight based on weight of resin solids.
The electrodepositable coating compositions of the present invention can be applied by electrodeposition to a variety of electroconductive substrates especially metals such as untreated steel, galvanized steel, conductive carbon coated materials, and aluminum. The applied voltage for electrodeposition may be varied and can be, for example, as low as 1 volt to as high as several thousand volts, but typically between 50 and 500 volts. The current density is usually between 0.5 ampere and 5 amperes per square foot and tends to decrease during electrodeposition indicating the formation of an insulating film.
After the coating has been applied by electrodeposition, it is cured usually by baking at elevated temperatures such as 90xc2x0 to 260xc2x0 C. for 1 to 40 minutes.
Illustrating the invention are the following examples which are not to be considered as limiting the invention to their details. Unless otherwise indicated, all parts and percentages in the following examples, as well as throughout the specification, are by weight.